Page 505 - Sensors and Control Systems in Manufacturing

458
Cha p te r
Ni ne

the greater the deviation of the slide from its reference position. The
difference is available as a digital value at the output of the processor.
After D/A conversion, the difference is presented as an analog value
to a final control element such as a transistor that triggers the drive
motor. The greater the deviation from the reference value, the greater
the motor speed and the adjustment speed of the slide. When there is
no further deviation, the motor stops. The motor, of course, has both
upper and lower speed limits. At lower speeds, the torque may
become so low that the tool may falter.
In an absolute control system with an incremental displacement
encoder, a sensor acts as a limit switch at the end position of the slide.
Once the machine is started, the slide is moved to the end position,
where an incremental displacement counter in the processor is set to
the reference point value. Each additional movement of the slide is
recorded by this counter so that, at any given time, it knows the abso-
lute position of the counter, like the absolute system with a displacement
encoder. The absolute control system with an incremental displace-
ment encoder is the most widely used today.

9.6.4 NC Software
As with a PLC system, it is necessary to draw up an accurate model
for an NC system before a program is written.
The machine must be told which movements it is to execute. For
this purpose, displacement data must be prepared that can be taken
from a drawing of the workpiece. In addition, the machine needs to
know what tools to use and when, the tool feed speed, and the cut-
ting speed. Certain additional functions such as the addition of lubri-
cants must also be programmed.
All these data are clearly of great importance for conventional
machining of workpieces. The data are compiled during the work
preparation in a factory to create a work plan for a skilled operator.
A conventional work plan is an excellent programming model.
However, work plans for manual operation cannot, as a rule, be con-
verted to NC operations.
To ensure there are no ambiguities in the description of displace-
ment data, there is general agreement about the use of Cartesian coor-
dinates for NC (Fig. 9.19). Different layers of the coordinate system
apply to different processing machines, as shown in Fig. 9.20.
The commands in an NC instruction set are made up of code let-
ters and figures. The program structure is generally such that sen-
tences containing a number of commands can be constructed. Each
sentence contains a sentence number. Table 9.3 shows an NC program
for an automatic drilling machine.
Displacement conditions are given the code letter G. The dis-
placement condition G00 is programmed in sentence 1, signifying
point-to-point control behavior. This condition thus also applies to all